<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-type" content="text/html;charset=UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
* {
  box-sizing:border-box;
}

.left {
  background-color:#2196F3;
  padding:20px;
  float:left;
  width:20%; /* The width is 20%, by default */
}

.main {
  background-color:#f1f1f1;
  padding:20px;
  float:left;
  width:60%; /* The width is 60%, by default */
}

.right {
  background-color:#4CAF50;
  padding:20px;
  float:left;
  width:20%; /* The width is 20%, by default */
}

/* Use a media query to add a break point at 800px: */
@media screen and (max-width:800px) {
  .left, .main, .right {
    width:100%; /* The width is 100%, when the viewport is 800px or smaller */
  }
}
</style>
</head>
<body>

<h2>Media Queries</h2>
<p>Resize the browser window.</p>

<p>Make sure you reach the breakpoint at 800px when resizing this frame.</p>

<div class="left">
  <p>Left Menu</p>
</div>

<div class="main">
  <p>Main Content</p>
</div>

<div class="right">
  <p>Right Content</p>
</div>

<center><a href="index.html">返回主页</a></center>
</body>
</html>

